The present invention relates to a semiconductor device including a gate electrode and a method for fabricating the device.
Recently, as semiconductor devices have been downsized drastically and as the number of devices integrated on a chip has been increasing steeply, dual-gate CMOSFETs (complementary metal-oxide semiconductor field-effect transistors) have been used more and more widely.
Hereinafter, a p-channel MOSFET included in a known dual-gate CMOS device will be described as a typical known semiconductor device with reference to FIG. 11.
As shown in FIG. 11, a gate electrode 3 of polysilicon is formed over a semiconductor substrate 1 of silicon with a gate insulating film 2 interposed between them. Normally, the gate electrode 3 is doped with a dopant, e.g., boron (B), by an ion implantation technique. The boron ions are implanted into a polysilicon film, of which the gate electrode 3 will be made, at an energy low enough to form a boron concentration profile in the gate electrode 3 with one of its peaks located near the upper surface thereof and to prevent the boron atoms from penetrating through the gate insulating film 2 into the semiconductor substrate 1.
In this case, if the polysilicon film to be the gate electrode 3 is annealed after having been doped with boron, the boron atoms in the polysilicon film diffuse toward the semiconductor substrate 1. Any inappropriate condition for the annealing process causes the boron atoms in the polysilicon film to permeate through the gate insulating film 2 in the semiconductor substrate 1. Then, the dopant concentration in the semiconductor substrate 1 changes to degrade the device characteristics. Also, where a metal layer is deposited on the polysilicon film to form a poly-metal gate electrode and then a silicon nitride film to be a hard mask is deposited on the metal layer and annealed or where a silicon nitride film to be a sidewall is deposited on the gate electrode 3 and annealed, the permeation of the boron atoms into the semiconductor substrate 1 is observed noticeably.
To suppress the boron atoms from permeating the semiconductor substrate 1, various measures have been taken; a silicon oxynitride film that can suppress the boron atom permeation to a certain degree is adopted as the gate insulating film 2.
However, even if the silicon oxynitride film is used as the gate insulating film 2, the boron atom permeation is not completely suppressible. Particularly, where the silicon oxynitride film is extremely thin (less than 3 nm, for example) to catch up with performance enhancement of devices, the silicon oxynitride film can suppress the boron atom permeation just slightly to say the least.